1. Field of the Invention
This invention pertains to retaining drilling fluid within the mud system of a drilling operation when there is a reduction or loss of pressure in the part of the mud circulating in the drill string or when the drill string is separated to add a new joint of pipe and specifically pertains to preventing spilling such fluid from the portion of the mud circulation system remaining in the part of the pipe assembly disconnected from the drill string when there is an addition of a new pipe joint.
2. Background of the Invention
A drilling operation of an oil or gas well generally involves a drill string with a drill bit attached to its lower end, a fluid system usually referred to as the "mud" system for lubricating the drill bit and for removing cutting debris from the well, and a drilling rig for supporting and rotating the drill string. The drill string is normally attached to the drilling rig component known as the "kelly", which is a longitudinal segment of drill stem that has a hexagonal or other discrete, multi-sided external surface for fitting into the central opening of the rotary table to allow the rotary table to rotate the kelly and, thus, the drill string attached and depending from it.
Each time that a new joint is added to the drill string, the drill string is separated from the kelly to allow the new connection to be made. The stand pipe from the mud system carries the drilling fluid to be circulated down through the drill string to a point above the kelly, which has been mentioned above is itself a rather long drill stem. When the connection is broken, unless provisions are made to include a "mud saver" valve of some sort, the drilling fluid or mud in the kelly is dumped onto the drilling rig floor, thereby causing a messy condition for the workmen, a possibly dangerous and hazardous condition, and creating an environmental spill that fouls the area. This occurs even though the drilling fluid circulation system itself is shut off because there is a great deal of the drilling fluid remaining in the kelly even when circulation is interrupted.
A sub outfitted with a mud-retaining device and referred to as a "mud saver" is known to have been employed in the prior art between the kelly and the drill string to prevent this dumping from happening. One such device that is in the marketplace is manufactured by National Oil Well. This sub incorporates a rubber tube of uniform wall thickness that collapses like a flattened hose to shut off mud flow. There is no internal supporting structure for the rubber tube, which fatigues rather rapidly and often fails to be satisfactory in retaining the drilling fluid even before failure of the hose because of the 180.degree. bend of the tube. That is, when a thick rubber tube is folded back on itself there is an inherent opening at the ends of the bend that allows fluid to escape. Thus, an unsatisfactory amount of fluid can escape at either end of bend of the flattened tube. In addition, the bend causes the rubber to be greatly stressed, eventually resulting in failure. Large chunks often tear or break off when this sub is used and fall from the disconnected sub or down hole through the drill string once circulation of drilling fluid is restarted.
Another device incorporating a pressurized "mud saver" check valve on a drill string has been patented by Torus Equipment of Oklahoma City, Okla. in U.S. Pat. No. 4,811,758. This device incorporates a uniform wall elastomeric element that is pressurized from the outside, which pressure is overcome by internal drilling fluid pressure under normal drilling fluid conditions. When the drilling fluid pressure drops, such as preliminary to separating the drill string to add another joint, the external pressure on the elastomeric element causes some elastic action to occur, but mainly causes one end of the element to slide longitudinally to permit the element to close off the bore of the sub. The elastomeric element buckles from only one direction, as with the National Oil Well sub described above. Therefore, as the end moves up to allow the collapsing to take place, the elastomeric element closes in a manner that is determined by how the element naturally gathers or folds together. Repeated gathering operations in this manner show that the elastomeric element is quickly fatigued, resulting in the same tearing apart problems associated with the National Oil Well sub.
A fabric or cord reinforced collapsible sleeve of uniform wall dimension is employed in the flow control valve assembly disclosed in U.S. Pat. No. 5,205,325 assigned to Piper Oilfield Products, Inc. of Oklahoma City, Okla.
Yet another mud saver device incorporating a collapsible elastomeric element is disclosed in U.S. patent application Ser. No. 08/094,513, filed Jul. 20, 1993, commonly assigned with the present application. This element includes three area lobes of reduced wall thickness dimension located at evenly spaced locations around the periphery of the element. The device is similar to the device of the '758 patent in that the element is externally pressurized by a pressure that is overcome by normal internal drilling pressure during drilling operations, but that is sufficient to collapse the element when internal pressure is reduced or removed. The collapsing comes from the three directions of the thin-wall lobes. In such operation, the collapsing walls bunch up somewhat and touch each other leaving three partial openings between the resulting merging cusps that are the last portions of the overall opening to close as the closing pressure differential builds up. As noted in that disclosure, a relatively high pre-charge pressure of approximately 450 psi is required to completely close the opening of the element that is employed in its mud saver.
Therefore, it is a feature of the present invention to provide an improved safety sub for retaining drilling fluids above the sub in the event of reduction or loss of drilling fluid pressure.
It is another feature of the present invention to provide an improved safety sub of the type described above wherein the pressurized elastomeric cartridge is externally pressurized at a relatively low pressure to cause closing of a non-sliding, elastomeric element.
It is yet another feature of the present invention to provide an improved safety sub of the type described above having a pressurized elastomeric cartridge that includes a single thin-wall area that controllably and supportably collapses in an improved manner to minimize wear of the elastomeric element.